There is a relatively new class of optical mice and other optical navigation input devices for computing applications. These optical devices facilitate tracking input movements on specular navigation surfaces such as glass tabletops which do not have substantial texture for imaging. In general, these optical navigation input devices rely on light scattered by small particles and scratches. This scattered light slightly increases the angular range of the otherwise collimated specular reflection off the glass surface. By capturing the scattered light off axis of the reflected beam using an offset imaging aperture (i.e., the imaging aperture is offset relative to the main intensity of the specular reflection of the incident light), images of such scattering sites can be projected on a pixel array in a sensor, which can then be used to determine the motion of the mouse relative to the tabletop.
One challenge to implementing a compact version of such a system is that light from the tails of the Gaussian beam can enter the imaging aperture and overwhelm the signal from surface scattering features. In other words, the peripheral light distribution around the main intensity of the reflected light can have a higher intensity than the intensity of the scattered light. For example, if a Gaussian beam with a diameter of 1.2 mm at the 1/e2 intensity points intersects a plane of an imaging aperture with a diameter of 0.8 mm, with a beam center to imaging aperture center offset of 1.5 mm, approximately 1/10,000 of the beam power will pass through the offset imaging aperture. At the edge of the offset imaging aperture, the intensity of the light will be approximately 1/1,000 of the intensity of the center of the beam. Although the intensity of the light at the imaging aperture is much less than the intensity of the light at the beam center, in this example, the intensity of the light at the imaging aperture is nevertheless at least two orders of magnitude too high for adequate detection of the relatively low intensity of the light scattered from features on the glass surface.
One way to reduce the intensity of the light leakage through the imaging aperture is to place a circular illumination aperture around the collimated laser beam as it leaves the optical source. In this configuration, the collimated light passes through the illumination aperture prior to illuminating the navigation surface. This truncates the Gaussian beam so that there is no light beyond a certain radius from the beam center. Unfortunately, diffraction at the illumination aperture causes the beam to diffract outwards as the beam propagates toward the navigation surface and reflects towards the plane of the imaging aperture. The intensity of the diffracted light varies based on the size of the illumination aperture. In one example, it may be possible to optimize the illumination aperture size (e.g., about 1.0 mm) for a specific configuration, based on the tradeoff between beam growth due to diffraction and the native intensity of the tails of the Gaussian beam. However, even using an optimized radius for a circular illumination aperture, the amount of power entering the imaging aperture (e.g., about 6 nW per 1 mW beam) can still be at least one order of magnitude too high.
Another possible approach to address this problem is to make the optical system larger, which allows more linear distance between the center of the reflected light beam and the center of the imaging aperture center, while still detecting light from the same angle relative to beam axis. However, making the optical system larger would result in making the computer mice and other optical navigation input devices larger. Larger devices are often less maneuverable and less portable.